( 614 ) 



of the I wo spectra. With n view to this adjustment care had been 

 taken that the two sj)ectra were as close above each other as i)os- 

 sible ') and had al)oiit the same height. Tlie adjustment and reading- 

 were made in tlie four ditFerent positions of iV/c^ whicii gave equality 

 of intensity. 



§ 3. O/m'rnition.s. Only observations above the critical temperature 

 have been communicated here; in order to get nnambiguous data 

 for the dependence of the intensity of the opalescence on the tem- 

 perature and the density below the critical temperature, a stirring- 

 apparatus, or an arrangement to keep the temperature constant till 

 the thermo-dynamic ecpiilibrium should have been reached, would 

 have been required. The observations were made after the tube of 

 ethylene had t)een kept at higher temperature for J 5 hours or longer, 

 andjiad then been slowly cooled down to the temperature of obser- 

 vation. The measurements have been made for two wavelengths, 

 corresponding to D and F in the solar spectrum''). In order to give 

 an ideaof the degree of accuracy of the adjustments, we have com- 

 municated the data of an observation at a mean intensity of the 



scattered light in table I. 



TABLE I. 



1) The use of a Hüfker's prism would render more accurate adjustments 

 possible. 



2) When the experiments are repeated under circumstances which admit of a 

 more accurate speclropholoaictric adjustment, an extension of the measurements 

 to more wavelengths will be desirable. 



